As already disclosed, for example, in the publication of JP-B No. 51-8424(1976), a process is known for producing a foamed body of thermoplastic resin by filling the cavity of a mold with the thermoplastic resin as melted and containing a foaming agent and thereafter enlarging the cavity. As shown in FIG. 12 by a line 91, the cavity is enlarged in this process at a rate of V2 for a period of time T2 after the completion of filling.
Further known is a process wherein the cavity is enlarged at different rates to obtain a foamed body having a desired feel to the touch by controlling the thickness of a nonexpanded skin layer as disclosed, for example, in the publication of JP-A No. 7-88878(1995). With this process, the cavity is enlarged at a rate of V1 until time T1 after the completion of filling and then at a lower rate V3 from time T1 to time T2.
The publications of JP-B No. 51-8424(1976) and JP-A No. 6-198668(1994) disclose a process wherein the cavity of a mold used is enlarged for forming a foamed body of thermoplastic resin. This process prepares an expanded molding by filling the cavity with the thermoplastic resin as melted and containing a foaming agent and thereafter forcing the cavity to enlarge rapidly by a predetermined amount.
The foamed body is produced by this process by filling the thermoplastic resin containing a foaming agent into the cavity as held diminished from the final shape thereof and subsequently enlarging the cavity to the size of the final product.
However, the conventional processes for producing foamed bodies of thermoplastic resin have the problem that it is difficult to obtain foamed bodies of great expansion ratio, for example, those having heat-insulating properties and an expansion ratio of at least 5 times. Another problem encountered is that the conventional process affords only those having a definite cell diameter as large as at least several millimeters, producing only hollow moldings in the case of unsuited conditions.
Additionally the conventional processes have the problem that the resin fails to fully expand in the cavity corners in conformity with the enlargement of the cavity. Stated more specifically, when a resin 104 containing a foaming agent is filled into a cavity 103 defined by a fixed mold 101 and a movable mold 102 as shown in FIG. 13(a) and when the cavity 103 is thereafter enlarged by moving the movable mold 102 toward the direction of arrow 107 as shown in FIG. 13(b), it is likely that the thermoplastic resin 104 will fail to expand in the corners of the cavity 103 in conformity with the enlargement of the cavity 103, consequently affording only a foamed body 106 which is recessed at corners 105.
An object of the present invention, which is accomplished in view of the foregoing problems of the prior art, is to overcome these problems and to provide a process for producing a foamed body of thermoplastic resin of desired shape having a high expansion ratio and containing fine cells of desired diameter, and a mold for use in the process for forming foamed bodies of thermoplastic resin.
On the other hand, the publication of JP-A No. 10-230528(1998) discloses a process for producing a foamed body having a surface layer of nonexpanded portion integral therewith and satisfactory in surface appearance and having fine closed cells with a uniform average cell density, using carbon dioxide or nitrogen in a supercritical state as a foaming agent.
The publication of JP-A No. 8-108440(1996) discloses an expanded board of polyolefin resin having a cell structure of closed cells (a) at least 85% of which are 2.5 to 10.0 in the ratio of the size thereof in a direction perpendicular to the plane of the board to the size thereof in a direction parallel to the board, and (b) at least 70% of which are up to 500 μm in size in a direction parallel to the plane of the board, the expanded board being at least 2 in the ratio of the compressive strength thereof in a direction perpendicular to the plane of the board to the compressive strength thereof in a direction parallel to the plane of the board, and 5 to 20 times in expansion ratio. This expanded board of polyolefin resin is produced from a mixture of the polyolefin resin, a chemical foaming agent and a crosslinking agent by applying pressure to the mixture using molds having opposed parallel inner surfaces, heating the mixture at a temperature not lower than the decomposition temperature of the foaming agent, and subsequently increasing the distance between the molds to expand the polyolefin resin only in a direction perpendicular to the inner surfaces through which the pressure is applied. This process involves ingenuity to produce a flatted cell structure to achieve both a low density and high compressive rigidity.
Since the production process of JP-A No. 10-230528(1998) is an injection molding process, the foamed body obtained is nevertheless low in compressive strength and in properties to absorb great impact because of its structure of fine and uniform closed cells although having shape conformability. The product therefore needs to be given a higher density so as to be improved in physical properties, whereas this entails the problem of making the molding heavier and impaired in heat-insulating properties.
The molding of JP-A No. 8-108440(1996) consists solely of an expanded layer providing a single-layer structure and is therefore low in flexural strength and small in deformation limit due to the presence of closed cells. The process as described above also has the problem of affording moldings only in the form of a board. The use of the chemical foaming agent results in an increased cost and permits the unreacted components and decomposed products pf the agent to remain in the foamed body, consequently entailing discoloration of the foamed body, release of odor therefrom and problems of food hygiene.
In view of the above situations, another object of the present invention is to provide a foamed body of thermoplastic resin which has high compressive rigidity and high flexural rigidity in the direction of thickness thereof, lightweight, outstanding in heat-insulating properties and impact absorbing properties and free from the residues of chemical foaming agent.